FIG. 5 shows a working view of a prior art braking device for a bicycle. As illustrated, when the upper cable 1 is pulled upward by the handle, the lower cable 11 will be also pulled upwards. Hence, the actuating seat 12 will be pulled to turn about pin 13 thereby forcing the shoe 14 to press on the rim 15 of a wheel. As shown in FIG. 6, the braking device includes an actuating seat 12, a shoe 14, and a packing 17 with an inclined surface. When it is desired to adjust the shoe 14, it is only necessary to rotate the packing 17 to an appropriate position. However, it is difficult to control the turning angle of the packing thereby causing much inconvenience in operation.